a. Field of the Invention
The invention relates to servo systems having electromagnetic actuators for moving and positioning transducers or the like, and in particular to an improved control circuit for a power driver used with such actuators.
b. Prior Art
Electromagnetic actuators are used in electronic closed loop servo systems to position a movable member, such as a read-write magnetic head, to a desired location. In random access data storage and retrieval applications involving rotating magnetic disks, such servo systems operate in two modes: a track seeking or coarse mode and a track following or fine mode. An example of the manner of operation of a servo system in the track seeking mode is found in U.S. Pat. No. 4,027,338, issued May 31, 1977 to R. Kril for Transducer Positioning System for Providing Coarse Positioning, while an example of the manner of operation of a servo system in the fine mode is found in U.S. Pat. No. 3,864,740, issued Feb. 4, 1975 to F. Sordello and J. Cuda for Track Following Servo System. Inasmuch as signals indicative of servo error may be forward or reverse (positive or negative), the actuator must be able to respond to either type of error. A typical actuator is described in U.S. Pat. No. 3,737,969, issued June 12, 1973 to I. Pejcha for Centering Device, showing a current carrying coil disposed between poles of a permanent magnet. The current carrying coil is known as a linear induction motor, or simply a motor, and is also known as an electromagnetic actuator (EMA). The coil magnetically drives a carriage to which a servo head arm assembly and a number of read-write head arm assemblies are mounted. Forward current initially applied to the coil accelerates the carriage forward and reverse current applied thereafter decelerates the forward motion. Conversely, reverse current initially applied to the coil accelerates the carriage backward and forward curent applied thereafter decelerates the backward motion.
Because of the requirement to provide the EMA with forward and reverse current, a bridge circuit of transistor switches, as shown in FIG. 1, has been provided as a convenient means of achieving the requirement. In this description of the bridge circuit, whenever any transistor switch is said to be on or closed, it means that the switching transistor is saturated with a negligible voltage drop between the collector and emitter of the transistor. On the other hand, whenever any transistor switch is said to be off or open, it means that the switching transistor is turned off and does not conduct. Coil 11 represents the entire EMA in this and the remaining figures and henceforth the coil will be termed the EMA. Transistor switches 13 and 15 form a pair of symmetrically opposed switches which are the lower arms of the bridge across the EMA. These switches, only one of which may be commanded to be open at a time, steer forward or reverse current through EMA 11. For example, when switch 13 is open and switch 15 is closed, forward current will flow through EMA 11. On the other hand, when switch 13 is closed and switch 15 is open, reverse current will flow through EMA 11. The switches 13 and 15 are supposed to change state simultaneously. Similarly, the transistor switches 17, 19 form a pair of symmetrically opposed switches which are a pair of upper arms for the electrical bridge to control the amount of current across EMA 11. The top of the bridge is connected to ground at node 21, while the bottom of the bridge is connected to a negative supply voltage at node 23. In parallel with each of the lower arms of the bridge are the diodes 23, 25, while in parallel with the upper arms of the bridge are the diodes 27, 29. Also arranged in parallel across upper arms of the bridge are the power resistors 31, 33, while a current sampling resistor 35 is placed in series with the EMA 11.
In the circuit of FIG. 1 it is important that the sequence of opening and closing switches is precisely controlled. For example, if switch 13 is not turned off before switch 17 at the same side of the bridge is turned on, a heavy current will flow from ground to the negative supply through switch 17 and switch 13 and is likely to burn out both of the switches in the process. In order to protect the integrity of the entire bridge, sequence control circuits 43, 45, 47 and 49 are provided. These sequence control circuits are analog control circuits which receive various commands to open switches in response thereto in a proper sequence. The driver switching sequence control circuits 43 and 45 respond to drive commands, reverse and forward respectively. The pulser switching sequence control circuits 47 and 49 respond to pulse commands, forward and reverse respectively. The sequence control circuits provide assurance that when one of the driver switches 13 or 15 is closed, the opposite member of the pair of driver switches forming the lower bridge arms is open, and simultaneously the pulser switch in the upper arm diagonally opposite to the closed drive switch in the lower arm is also closed, while the other pulser switch is open. Thus, if switch 13 is closed, switch 19 is also closed while switches 15 and 17 are both open. Because the switching speed of transistor switches is sometimes difficult to predict and control, there is a continuing problem of preventing switch burnout in the process of driving EMA 11.
Another problem with the power driver control circuit of FIG. 1 is that the two low resistance power resistors 31 and 33 which are included in the circuit to provide a certain constant current through the EMA during the fine mode of operation consume relatively large amounts of power in both coarse and fine modes of operation. In the case when a 48 volt supply is used as shown, each 100 ohm resistor would continuously consume 23 watts. Therefore high power resistors rated at tens of watts are required.
Still another problem with the power driver control circuit of FIG. 1 is that current flows through the current sensing resistor in both directions, therefore a complicated current sensing circuit 37 which generates a current sample through the current-to-voltage converter 39 is required. The complexity of the required circuit causes errors in the reported current sample, especially in reporting similar currents from one direction to the other, thereby adversely influencing servo control.